U.S. patent number 6,299,774 [Application Number 09/602,684] was granted by the patent office on 2001-10-09 for anaerobic digester system.
Invention is credited to Jack L. Ainsworth, Dan Atwood, Tom Rideout.
United States Patent |
6,299,774 |
Ainsworth , et al. |
October 9, 2001 |
Anaerobic digester system
Abstract
A process to recover energy, reduce pollution potential, and add
value to organic waste such as animal manure is described. The
process involves the anaerobic digestion of feedstocks, such as
animal manure, at low to high temperatures in batch,
semi-continuous or continuous reactors. The process makes use of
existing handling and storage equipment at the farm and requires
minimal supervision and skill by the operator. The system is not
affected by high concentrations of volatile acids and ammonia or
nitrogen. The productivity of the anaerobic digester system, in
terms of methane a gas production and quality, is exceptionally
high. The anaerobic digester requires only a singe reaction vessel.
Consequently, the process is low cost and does not interfere with
regular farm operations.
Inventors: |
Ainsworth; Jack L. (Canton,
TX), Atwood; Dan (Nassau Bay, TX), Rideout; Tom
(Midland, TX) |
Family
ID: |
24412358 |
Appl.
No.: |
09/602,684 |
Filed: |
June 26, 2000 |
Current U.S.
Class: |
210/603; 210/178;
210/180; 210/218; 210/612; 210/632; 435/290.2; 71/23; 71/21; 71/19;
71/16; 71/15; 71/10; 435/262.5; 210/613; 210/221.2; 210/205 |
Current CPC
Class: |
C12M
21/04 (20130101); C12M 41/22 (20130101); C02F
3/28 (20130101); C12M 41/40 (20130101); C12M
45/02 (20130101); C12M 47/14 (20130101); C02F
3/34 (20130101); C12M 47/20 (20130101); C12M
45/20 (20130101); C12M 27/06 (20130101); C12M
47/18 (20130101); C05F 17/40 (20200101); Y02W
30/43 (20150501); Y02P 20/145 (20151101); Y02E
50/343 (20130101); C02F 11/04 (20130101); Y02W
30/47 (20150501); Y02W 10/20 (20150501); Y02E
50/30 (20130101); Y02P 20/59 (20151101); Y02W
10/37 (20150501); Y02W 30/40 (20150501); Y02W
10/23 (20150501) |
Current International
Class: |
C05F
17/00 (20060101); C02F 3/28 (20060101); C12M
1/107 (20060101); C02F 3/34 (20060101); C02F
11/04 (20060101); C02F 003/28 (); C02F 003/34 ();
C05F 001/00 (); C05F 003/02 (); C05F 015/00 () |
Field of
Search: |
;210/603,612,613,632,178-180,198.1,205,209,218,220,221.1,221.2
;435/262,262.5,290.1,290.2 ;71/11-24,10 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Simmons; David A.
Assistant Examiner: Prince; Fred
Attorney, Agent or Firm: Matos; Rick Innovar, L.L.C.
Claims
We claim:
1. An anaerobic digester system for converting cellulose-containing
feedstock into useful materials, wherein the system comprises:
a feedstock slurry feeder;
a plurality of conduits connecting various components of the
system;
a single pressurizable anaerobic digester comprising agitation
means, one or more feed ports, one or more discharge ports, and a
reaction vessel for holding a reaction solution comprising an
anaerobic microbe which converts an aqueous slurry of
cellulose-containing feedstock into at least one useful gas and an
enriched effluent;
a pressurizer; and
one or more gas processors directly or indirectly connected to the
anaerobic digester;
wherein the headspace of the anaerobic digester is pressurized to
at least about 10 psi to form the enriched effluent and the at
least one useful gas during anaerobic digestion of the
cellulose-containing feedstock.
2. The anaerobic digester system of claim 1 further comprising one
or more of the following:
a) one or more pressure controllers for controlling the pressure
inside the anaerobic digester; and
b) one or more temperature controllers for controlling the
temperature inside the anaerobic digester.
3. The anaerobic digester system of claim 2, wherein at least one
of the one or more pressure controllers comprises an actuator for
the pressurizer and a pressure monitor.
4. The anaerobic digester system of claim 2, wherein at least one
of the one or more temperature controllers is selected from a coil
inside the anaerobic digester, a jacket outside the reaction
vessel, and a combination thereof.
5. The anaerobic digester system of claim 4, wherein at least one
of the one or more temperature controllers further comprises a
recirculating fluid source outside the reaction vessel.
6. The anaerobic digester system of claim 5, wherein the
recirculating fluid source is selected from the group consisting of
a solar water heater, a gas-powered water heater, a steam
generator, and a heat exchanger.
7. The anaerobic digester system of claim 1 further comprising one
or more vessels for preparing a feedstock slurry.
8. The anaerobic digester system of claim 1 further comprising a
feedstock particle size reducer.
9. The anaerobic digester system of claim 1 further comprising one
or more storage tanks for receiving the enriched effluent.
10. The anaerobic digester system of claim 9, wherein the one or
more storage tanks includes a scum storage tank, a supernatant
storage tank and a sludge storage tank.
11. The anaerobic digester system of claim 1, wherein the one or
more gas processors is selected from the group consisting of a gas
scrubber, a gas compressor, a gas separator, a gas treater, and an
aerial cooler.
12. The anaerobic digester system of claim 1, wherein the agitation
means is selected from the group consisting of a fluid
recirculator, a gas recirculator, a sparger bar, a mechanical
agitator and a combination thereof.
13. The anaerobic digester system of claim 1, wherein the
pressurizer is one or more of the feedstock slurry feeder; an
external compressed gas source; a fluid inlet; a sparger bar, a gas
recirculator; and a fluid recirculator.
14. The anaerobic digester system of claim 1, wherein the at least
one useful gas is selected from the group consisting of methane,
ammonia, hydrogen, hydrogen sulfide and carbon dioxide.
15. The anaerobic digester system of claim 1, wherein the enriched
effluent comprises at least one of a nitrogen rich fertilizer, a
feed additive, an insecticidal mixture, ammonia, and an organic
acid.
16. A useful material prepared with the anaerobic system of claim
1, wherein the useful material comprises methane, ammonia,
hydrogen, hydrogen sulfide, carbon dioxide, a nitrogen rich
fertilizer, a feed additive, an insecticidal mixture, an organic
acid, or a combination thereof.
17. An anaerobic digester system for converting
cellulose-containing feedstock into useful materials, wherein the
system comprises:
a feedstock slurry feeder;
a plurality of conduits connecting various components of the
system;
a single pressurizable anaerobic digester comprising agitation
means, one or more feed ports, one or more discharge ports, and a
reaction vessel for holding a reaction solution comprising an
anaerobic microbe which converts an aqueous slurry of
cellulose-containing feedstock into at least one useful gas and an
enriched effluent;
a pressurizer;
one or more gas processors directly or indirectly connected to the
anaerobic digester; and
a gas recirculation system comprising a gas separator for reducing
methane from a discharge gas received directly or indirectly from
the anaerobic digester to form a methane-reduced gas which is
subsequently fed directly or indirectly back into the anaerobic
digester;
wherein the headspace of the anaerobic digester is pressurized to
at least about 10 psi to form the enriched effluent and the at
least one useful gas during anaerobic digestion of the
cellulose-containing feedstock.
18. The anaerobic digester system of claim 1 further comprising a
fluid recirculation system for adding scum, sludge, supernatant or
effluent directly or indirectly back into the anaerobic
digester.
19. The anaerobic digester system of claim 1, wherein the
configuration of the anaerobic digester system permits batch,
semi-continuous or continuous operation.
20. The anaerobic digester system of claim 1, wherein the agitation
means comprises a gas bubbler, an aerator, a sparger bar, a fluid
stream, a mechanical agitator, or a combination thereof.
21. The anaerobic digester system of claim 1, wherein the anaerobic
microbe is a methanogenic bacterium.
22. The anaerobic digester system of claim 21, wherein the
anaerobic microbe is psychrophilic or thermophilic.
23. The anaerobic digester system of claim 1, wherein the feedstock
slurry comprises from about 1% wt. to about 90% wt. solids.
24. The anaerobic digester system of claim 1 further comprising at
least one of an internal combustion engine, an electrical current
generator, an electric engine, a water heater, a furnace, an air
conditioning unit, a ventilation fan, a conveyor, a pump, a heat
exchanger, and a fuel cell.
25. The anaerobic digester system of claim 24; wherein the at least
one useful gas is used to operate at least one of the internal
combustion engine, electrical current generator, electric engine,
water heater, furnace, air conditioning unit, ventilation fan,
conveyor, pump, heat exchanger, and fuel cell.
26. The anaerobic digester system of claim 1, wherein the anaerobic
microbe is selected from the group consisting of:
Clostridium spp., Bacillus spp., Escherichia spp., Staphylococcus
spp., Methanobacter spp., Methanobacter (Mb.) omlianskii, Mb.
formicicum, Mb. soehngenii, Mb. thermoautrophicum, Mb. ruminatium,
Mb. mobile, Mb. methanica, Methanococcus (Mc.) mazei, Mc.
vannielii, Ms. mazei, Mb. suboxydans, Mb. propionicum,
Methanosarcina (Ms.) bovekeri, Ms. methanica, Ms. Alcaliphilum, Ms.
acetivorans, Ms. thermophilia, Ms. barkeri, Ms. vacuolata,
Propionibacterium acidi-propionici, Saccharomyces cerevisae, S.
ellipsoideus, Clostridium propionicum, Clostridium
saccharoacetoper-butylicum, and Clostridium butyricum.
27. The anaerobic digester system of claim 26, wherein the at least
one useful gas is selected from the group consisting of:
methane, hydrogen, carbon dioxide, hydrogen sulfide, and
ammonia.
28. The anaerobic digester system of claim 1, wherein the enriched
effluent comprises at least one of ammonia; nitrogen rich
fertilizer; protein; amino acid; carbohydrate; insecticide;
mineral; charcoal; carbon black; and insect repellant.
29. The anaerobic digester system of claim 1, wherein the aqueous
slurry of cellulose-containing feedstock comprises at least one of
animal tissue, biomass, fish tissue, plant parts, fruits,
vegetables, plant-processing waste, animal-processing waste, animal
manure, animal urine, mammalian manure, mammalian urine, solids
isolated from fermentation cultures, bovine manure or urine,
poultry manure or urine, equine manure or urine, porcine manure or
urine, wood shavings or chips, slops, mostos, shredded paper,
cotton burrs, grain, chaff, seed shells, hay, alfalfa, grass,
leaves, sea shells, seed pods, corn shucks, weeds, aquatic plants,
algae, fungus, and combinations thereof.
30. The anaerobic digester system of claim 1, wherein the system is
installed in a swine ranch, chicken farm, cattle ranch, feedlot or
dairy cattle milking facility to convert waste material into at
least methane, which is used to operate one or more gas-powered
machines.
31. A method of converting an aqueous slurry of
cellulose-containing feedstock into at least one useful gas and an
enriched effluent, the method comprising the steps of:
operating an anaerobic digester system according to any one of
claims 1-31 to produce the at least one useful gas and the enriched
effluent.
Description
FIELD OF THE INVENTION
The present invention relates generally to an improved process and
equipment for converting feedstock into useful materials, and more
specifically, to an anaerobic fermentative process for
bioconverting waste, sewage sludge or other biodegradable feedstock
into methane gas, carbon dioxide gas, ammonia, carbon black, an
organic acid, charcoal, a fertilizer and/or an insecticidal
mixture.
BACKGROUND OF THE INVENTION
Animal waste poses a significant problem in the poultry, swine and
cattle industries. Animal waste from animal raising or processing
operations is responsible for a significant amount of underground
water contamination and methods are continually being developed for
handling animal wastes. One known method is the bioconversion of
animal waste into useful products.
Methods for the anaerobic digestion or treatment of sludge, animal
waste, synthesis gas or cellulose-containing waste are disclosed in
U.S. Pat. No. 5,906,931 to Nilsson et al., No. 5,863,434 to Masse
et al., No. 5,821,111 to Grady et al. No. 5,746,919 to Dague et
al., No. 5,709,796 to Fuqua et al., No. 5,626,755 to Keyser et al.,
No. 5,567,325 to Townsley et al., No. 5,525,229 to Shih, No.
5,464,766 to Bruno, No. 5,143,835 to Nakatsugawa et al., No.
4,735,724 to Chynoweth, No. 4,676,906 to Crawford et al., No.
4,529,513 to McLennan, No. 4,503,154 to Paton, No. 4,372,856 to
Morrison, No. 4,157,958 to Chow, and No. 4,067,801 to Ishida et al.
These patents disclose different processes and equipment for the
bioconversion, either by microbial digestion or enzymatic
conversion, of those materials into methane and other useful
materials.
The equipment used for the anaerobic digestion or fermentation of
waste into fuel, such as methane, varies greatly and is generally
tailored to specific applications. Equipment that is suitable for a
first type of feedstock generally has to be modified before it can
be used for a second different type of feedstock.
Chemical and biochemical reactions that create a gas are generally
conducted at low to subatmospheric pressures due to the tendency of
the product gas to function as feedback inhibitor that inhibits
further formation of the gas. The art recognizes that variations in
the pressure of an anaerobic digester can be used to effect
different biochemical and productivity results. U.S. Pat. No.
4,409,102 to Tanner discloses an anaerobic digestion conducted at
subatmospheric pressures which unexpectedly effects an increase in
methane gas production. U.S. Pat. No. 3,994,780 to Klass et al.
discloses the high pressure rupture of cells in an anaerobic
digester to render cellular components available to other intact
cells in the digester. U.S. Pat. No. 3,981,800 to Ort discloses a
process for preparing high quality methane (about 98% wt.) with an
anaerobic digester operated at 1-5 atm. above atmospheric pressure
provided that the sludge is degassed by a recirculator and passed
between two digesters to remove carbon dioxide in the sludge which
is then fed back into the digester. U.S. Pat. No. 4,100,023 to
McDonald discloses that the internal pressure of the anaerobic
digester should be kept at about 1 to 3 inches of water column to
ensure proper performance. U.S. Pat. No. 4,568,457 to Sullivan
discloses a two-stage anaerobic digester system having an acid
forming stage and a methane gas forming stage, wherein the pressure
of the gas in the headspace of the two stages can be slightly above
atmospheric pressure.
Methanogenic microbes that create methane from carbon and hydrogen
containing feedstocks, such as cellulose, animal waste, food
processing waste, and sludge, are well known. These microbes have
been used in the waste processing industry and are available in
their native forms from natural sources or in genetically altered
or manipulated forms, which can produce greater amounts of useful
materials per unit weight of waste than can unaltered methanogenic
bacteria.
To date, no equipment containing the required components as
described herein has been disclosed. Further, the improved
equipment design and layout of the present invention provides a
higher yield of methane and other useful materials than other
comparable equipment. Still further, the improved process and
equipment of the invention can be used in the poultry, swine, dairy
or cattle industries to convert cellulose-containing animal waste
into methane which is used to operate farm or ranch equipment
thereby reducing operating costs and the volume of waste
produced.
SUMMARY OF THE INVENTION
The present invention provides a system for converting
cellulose-containing feedstock into useful materials, wherein the
system comprises:
a feedstock slurry feeder,
a plurality of conduits connecting various components of the
system;
a single pressurizable anaerobic digester comprising agitation
means, one or more feed ports, one or more discharge ports, an
optional pressure regulator, and a reaction vessel for holding a
reaction solution comprising an anaerobic microbe which converts an
aqueous slurry of cellulose-containing feedstock into at least
methane and an enriched effluent;
a pressurizer; and
one or more gas processors directly or indirectly connected to the
anaerobic digester; wherein the headspace of the anaerobic digester
is pressurized to about 10 psi or more to form the enriched
effluent and a discharge gas comprising at least methane during
anaerobic digestion of the feedstock slurry.
Depending on the feedstock slurry used, the anaerobic digester will
also form a fertilizer, sludge, scum, ammonia, charcoal, carbon
black, an organic acid and/or an insecticidal mixture. The
anaerobic digester is preferably operated at pressures between 10
to 265 psi, more preferably 10 to 100 psi, and even more preferably
25-75 psi. In preferred embodiments, the system also comprises one
or more of the following: one or more gas scrubbers, one or more
heaters for heating or preheating the slurry being digested in the
anaerobic digester, one or more water storage tanks, one or more
feedstock slurry tanks, one or more feedstock grinders, one or more
supernatant storage tanks, one or more sludge storage tanks, one or
more sludge dryers, one or more scum storage tanks, one or more
CO.sub.2 tanks, and/or one or more produced gas storage tanks.
Other preferred embodiments include those wherein the system does
not require a water lagoon, a foam trap, and/or a water vapor trap.
Still other preferred embodiments include those wherein: (1) the
system is operated in a batch, semi-continuous, or continuous mode;
(2) the feedstock slurry comprises from about 1-90% wt. solids,
more preferably about 1-60% wt. solids, or even more preferably
about 1-40% wt. solids; (3) the agitation means comprises a gas
bubbler, an aerator, a sparger bar, a fluid stream, a mechanical
agitator, or a combination thereof; (4) the feedstock slurry is
gravity fed or fed under pressure to the anaerobic digester; (5)
the pressurizer pressurizes the anaerobic digester with gas or a
liquid; (6) the pressurizer is the feedstock slurry feeder, which
is preferably a pump, gravity feed system, or a gas compressor; (7)
the anaerobic digester does not require aerobic digestion of the
feedstock; (8) the anaerobic digester does not require multiple
discrete zones of environmentally incompatible waste-digestive
microorganisms; (9) the anaerobic microbe is a methanogenic
bacterium; (10) the anaerobic microbe is psychrophilic or
thermophilic; (11) methane produced by the anaerobic digester is
used to operate an internal combustion engine, for example for
driving an electrical current generator or an electric engine, or
the methane gas can be used to operate a water heater, a furnace,
an air conditioning unit, a ventilation fan, a conveyor, a pump, a
heat exchanger, fuel cell, or various components of the system
itself and/or to recharge power cells; (12) the gas processor
comprises a gas scrubber and/or a gas separator, (13) a gas
recirculator is used to recirculate gas from the headspace of the
reactor to the slurry in the reactor; (14) a gas recirculator adds
methane-depleted or carbon dioxide enriched biogas back to the
reactor; and/or (15) a fluid recirculator recycles the scum,
supernatant, effluent, or sludge of the reactor.
Other features, advantages and embodiments of the invention will be
apparent to those skilled in the art by the following description,
accompanying examples and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings are part of the present specification and
are included to further demonstrate certain aspects of the
invention. The invention may be better understood by reference to
one or more of these drawings in combination with the detailed
description of the specific embodiments presented herein.
FIGS. 1a and 1b are process flow schematics of a first preferred
embodiment of the anaerobic digester system according to the
invention.
FIG. 2 is a process flow diagram of a second preferred embodiment
of the anaerobic digester system of the invention.
FIG. 3 is a chart depicting the temperature, pH, pressure and
methane gas volume production of an exemplary digester according to
the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is different than known anaerobic digester
system primarily in that it is conducted at elevated pressures of
at least about 10 psi up to about 265 psi, more preferably 10 to
100 psi, and even more preferably 25-75 psi, during anaerobic
digestion of a feedstock slurry and the system requires only a
single anaerobic digester. The anaerobic digester system also
includes an advantageous combination of known and unknown features
that unexpectedly provides a very efficient system for converting
biomass into methane gas, a nutrient enriched solution, and
optionally an insecticidal mixture.
As used herein, the term "feedstock" is taken to mean any animal or
plant derived material that contains one or more components that
can be converted, bioconverted or biodegraded into a useful
material by the anaerobic digester of the invention. Animal tissue,
biomass, fish tissue or parts, plant parts, fruits, vegetables,
plant processing waste, animal processing waste, animal manure or
urine, mammalian manure or urine solids isolated from fermentation
cultures, and combinations thereof are included in the term
feedstock. Particular examples of feedstock include bovine,
poultry, equine or porcine manure or urine, wood shavings or chips,
slops, mostos, shredded paper, cotton burrs, grain, chaff, seed
shells, hay, alfalfa, grass, leaves, sea shells, seed pods, corn
shucks, weeds, aquatic plants, algae and fungus and combinations
thereof. Combinations of poultry, bovine, equine or porcine urine
and/or manure with wood shavings, wood chips, shredded paper,
cotton burrs, seed shells, hay, alfalfa, grass, leaves, seed pods,
or corn shucks are particularly preferred and are generally
referred to as cellulose-containing feedstocks.
A feedstock slurry is prepared by suspending a feedstock in an
aqueous solution to form a slurry comprising less than about 90% wt
solids, preferably about 0.1-60% wt solids, or even more preferably
about 1-40% wt. solids. The particle size of the feedstock can be
reduced either prior to or during preparation of the feedstock
slurry by employing an in-line or immersed abrader, classifier,
mill, high shear mixer, grinder, homogenizer or other particle size
reducer known to those of ordinary skill in the art. No particular
particle size is required for the feedstock; however, smaller
particle sizes are preferred as smaller particles are generally
bioconverted more quickly than larger particles.
Grit, such as dirt, sand, soil, stones, pebbles, rocks, feathers,
hair and other such materials, is preferably removed prior to
addition of the feedstock slurry to the anaerobic digester;
however, grit can be removed at any point along the process.
Equipment such as classifiers, settling tanks, multiphase tanks,
and/or or filters can be used to remove the grit.
As used herein, the term "useful material" is taken to mean methane
gas; hydrogen gas; carbon dioxide; hydrogen sulfide; nitrogen rich
fertilizer, protein, amino acid, carbohydrate and/or mineral rich
solution or slurry; insecticidal mixture; charcoal; carbon black;
insect repellant mixture; combinations thereof and other such
materials which can be prepared by anaerobic digesters from a
feedstock. Methane, a nitrogen rich fertilizer, and an insecticidal
slurry are particularly preferred useful materials.
The anaerobic microbe used in the anaerobic digester is any
anaerobic bacterium, fungus, mold or alga, or progeny thereof,
which is capable of converting the feedstock to a useful material
in the anaerobic digester of the invention. Preferred anaerobic
microbes are isolated from decaying or composted feedstock, are
endogenous to the area in which the feedstock was first obtained,
are obtained from bacterial or fungal collections such as those of
the American Type Culture Collection (ATCC) or have been
genetically altered or engineered to convert a feedstock to a
useful material. Particularly preferred anaerobic microbes are
those which will convert a cellulose-containing feedstock into
methane, a nitrogen rich fertilizer, humus and an insecticidal
slurry. The anaerobic microbe can be a psychrophile, mesophile or
thermophile.
Examples of an anaerobic microbe which is useful in the anaerobic
digester of the invention include yeast, a methanogenic bacterium,
methanobacterium acetobacterium, acetogenic bacterium, liquefaction
bacterium, Clostridium spp. (methane), Bacillus spp., Escherichia
spp., Staphylococcus spp., Methanobacter s pp., Methanobacter (Mb.)
omlianskii (methane), Mb. formicicum (methane), Mb. soehngenii
(methane), Mb. thermoautrophicum (methane), Mb. ruminatium
(methane), Mb. mobile (methane), Mb. methanica (methane),
Methanococcus (Mc.) mazei (methane), Mc. vannielii (methane), Ms.
mazei (methane), Mb. suboxydans (methane), Mb. propionicum
(methane), Methanosarcina (Ms.) bovekeri (methane), Ms. methanica
(methane), Ms. alcaliphilum (methane), Ms. acetivorans (methane),
Ms. thermophilia (methane), Ms. barkeri (methane), Ms. vacuolata
(methane), Propionibacterium acidi-propionici (methane),
Saccharomyces cerevisae (ethanol), S. ellipsoideus (ethanol),
Clostridium propionicum (propanol), Clostridium
saccharoacetoper-butylicum (butanol), Clostridium butyricum
(hydrogen), wherein the chemical in parentheses indicates a useful
material which that microbe produces.
Other microbes and/or enzymatic catalysts can be added to the
anaerobic digester to facilitate breakdown of the feedstock into
components which are usable by the anaerobic microbe as either
nutrients or starting materials for useful materials made by the
anaerobic microbe. Such other microbes and/or enzymes include, for
example, amylases, proteases, cellulases, hydrolases, lipid
hydrolyzing enzymes, lysozymes, phosphatases, esterases, amidases,
and lipases.
The conditions inside the anaerobic digester will vary according to
the useful material being produced, the anaerobic microbe being
used, the configuration of the anaerobic digester, the feedstock
being converted, the desired productivity of the anaerobic
digester, and the form of microbe (immobilized or free-flowing)
used. Immobilized microbes can be prepared using any methods known
by the artisan of ordinary in the arts. The conditions used to
culture the anaerobic microbe and maintain it viable in the
anaerobic digester can be varied. Conditions which can be
controlled include solids content, reaction solution composition,
temperature, gas content, digestion rate, anaerobic microbe
content, agitation, feed and effluent rates, gas production rate,
carbon/nitrogen ratio of the feedstock, pressure, pH, and retention
time in the digester, among other things.
The amount of solids in the digester will generally range from
about 1 to about 60% wt., preferably from about 20 to about 50%
wt., more preferably about 40 to about 50% wt. based upon the total
solution weight.
The particle size of solids in the digester affects the rate of
digestion. Generally, the smaller the particle size, the faster the
rate of digestion.
The temperature of the reaction solution is generally in the range
of about ambient temperature, i.e., the temperature outside of the
digester, to about 200.degree. F., preferably about 70.degree. F.
to about 200.degree. F., more preferably about 80.degree. F. to
about 150.degree. F., even more preferably about
90.degree.-115.degree. F. The optimum operating temperature will
depend upon the anaerobic microbe used or the product being
produced. For Clostridium spp., the preferred temperature is in the
range of about 70-100.degree. F., and more preferably in the range
of about 70-95.degree. F., and most preferably in the range of
about 75-95.degree. F.
The feed rate of the anaerobic digester is expressed in terms of
lbs. of feedstock slurry added to the digester per unit time. The
feed rate can be varied as desired; however, for a 1000 gal reactor
maintained at approximately 80% of capacity, operated a temperature
of about 95 to 105 F., and being used to produce methane, about 50
to about 55 lbs. of poultry waste, containing 25% wt. poultry
manure and 75% wt. cotton burrs, about 1/40 of the total weight per
hour can be added to the digester.
The effluent rate, i.e., the rate at which effluent is drawn from
the digester, is related to the feed rate of the digester.
Generally, the effluent rate will not exceed the feed rate when the
digester is operated in a continuous mode. However, the feed rate
and effluent rate are generally independent of one another when the
digester is operated in a batch or semi-continuous mode. Further,
the total amount of feedstock slurry added to the digester will
generally exceed the total amount of effluent withdrawn from the
digester, since part of the feedstock is converted to a gas that is
also drawn from the digester. During continuous operation,
feedstock is continuously added to the reactor at approximately the
same time that gas, effluent, scum, supernatant and/or sludge are
removed from the reactor. During semi-continuous operation,
feedstock is added to the reactor incrementally and gas, effluent,
scum, supernatant and/or sludge are removed incrementally at the
same or different times. During batch operation, larger portions of
feedstock are added to the reactor at given time intervals and
larger portions of gas, sludge, effluent, supernatant and/or sludge
are removed from the reactor at the same or different time
intervals. During continuous operation, the operating temperature
and rate of gas production will be relatively constant. Generally
continuous operation will provide a greater rate of gas production
than batch or semi-continuous operation.
Gas production rate is expressed in terms of volume of fuel gas
produced per given time interval of operation, e.g. ft..sup.3 of
fuel gas produced per hour or day of operation, in terms of volume
of fuel gas produced per unit weight of feedstock added to the
reactor. In the example described herein, the digester produced
approximately 5-8 ft..sup.3 of methane per pound of feedstock.
The quality of the fuel gas produced is generally expressed in
terms of the BTU rating of the gas as it is removed from the
reactor. In the example described herein, the methane collected
from the digester had an average rating of about 500-800 BTU
without a recirculation loop installed. Higher ratings in the range
of about 800 to about 1000 BTU can be achieved using one or more of
the preferred embodiments described herein. Pure methane, or sweet
dry methane, has a rating of 1000 BTU.
A gas, such as methane, carbon dioxide, hydrogen, ammonia or
hydrogen sulfide, which is produced in the anaerobic digester may
be present in the reaction solution and headspace above the
reaction solution. The content of each gas in the headspace and the
reaction solution will vary according to the conditions, feedstock
and/or anaerobic microbe present within the anaerobic digester. The
content or percentage of each gas can be monitored using a gas
chromatograph or other gas sensing or analyzing equipment used to
determine the composition or presence of gases or gaseous mixtures.
In preferred embodiments for producing methane, the content of the
gas in the headspace will be about 60-100% methane, 0-40% carbon
dioxide and 0-10% of other gases, such as ammonia, hydrogen or
hydrogen sulfide. Since the digester is operated under
approximately or strictly anaerobic conditions, the content of
oxygen in the digester will preferably be less than about 10%, more
preferably less than about 1% and most preferably about 0%.
The methane, carbon dioxide, or hydrogen produced by the anaerobic
digester will generally be cleaned or purified by a scrubber to
remove moisture, vapor, droplets, suspended solids or other such
contaminants. The scrubber can comprise one or more of a filter,
desiccant, zeolite, activated carbon, fiber, countercurrent wash
solution, mixer, homogenizer, or other such components typically
used in association with or comprised within gas scrubbers. Such
components are well known to those of ordinary skill in the art of
gas processing. In general, hydrogen sulfide is an undesired
by-product or off-gas, which is removed from the desired product
gas.
The gases which exit the anaerobic digester or the scrubber are
then optionally separated into their individual components using
conventional gas separation equipment, which is known to those of
skill in the art for separating gas mixtures. The gases may also be
processed with one or more compressors, or dehydration equipment.
Alternatively, the gases are stored in pressurized storage vessels
or tanks once they have been scrubbed. If the stored gas is
purified methane or hydrogen or mixtures of methane or hydrogen
with carbon dioxide, it can be used directly to operate the
anaerobic digester or one or more of its components or it can be
used to operate additional equipment such as that described above.
Ammonia may also be found in the above-described gases.
The agitation means will agitate the reaction solution in the
reaction vessel. Exemplary agitation means include one or more
sparger bars, one or more mechanical agitators, a fluid
recirculator, a gas recirculator and combinations thereof.
The sparger bar will bubble a gas through the reaction solution.
The gas is preferably an inert gas, but it can also be a gas, such
as CO.sub.2, that is produced by the anaerobic digester system. The
gas source can be the gas in the headspace of the anaerobic
digester, gas that is downstream from the anaerobic digester, or a
gas cylinder. A preferred sparger bar will recirculate downstream
gas, and preferably gas that has had at least some of its methane
removed therefrom, back into the reaction vessel. By feeding back
into the reaction solution, in particular the sludge layer thereof,
a product gas that has had methane removed from it, the reactor
will produce more methane per pound of feedstock and the methane
will be of higher quality, i.e., it will contain less carbon
dioxide and have a higher BTU rating. In a preferred embodiment, a
gas recirculator will comprise a sparger bar for adding a
methane-stripped or reduced product gas, such as CO.sub.2, back
into the anaerobic digester, an in let port for receiving gas from
the anaerobic digester, and one or more pumps and/or gas
separators.
Another preferred embodiment of the invention provides an anaerobic
digester system comprising a gas recirculation system comprising a
gas separator for removing methane from the discharge gas received
directly or indirectly from the anaerobic digester to form a
methane-reduced gas, or carbon dioxide enriched gas, which is
subsequently fed directly or indirectly back into the anaerobic
digester. In this manner, the thermodynamic equilibrium for the
digestion of the feedstock is pushed toward methane production and
carbon dioxide consumption.
Another preferred sparger bar will recirculate gas from the
headspace of the reactor back through the reaction solution and
preferably the sludge layer to improve conversion of carbon dioxide
to methane.
A fluid recirculator will preferably recirculate reaction solution
from a first part of the reaction vessel to a second part of the
reaction vessel. Alternatively, the fluid recirculator will
recirculate feedstock slurry, scum sludge, supernatant or reaction
effluent, or portions thereof through the reaction vessel. For
example, the recirculator could recirculate either one or more of
the scum, supernatant or sludge phases of the reaction effluent. A
recirculator could also recirculate one or more fluids removed from
the digester and added to a tank back to the digester. A
recirculator could also recycle supernatant into the feedstock
feeder to aid in preparing the feedstock slurry. According to
another preferred embodiment, a fluid that is recycled back into
the reaction vessel will have been stripped of at least some and
preferably most or all of its methane gas prior to being added back
to the reaction vessel.
Mechanical agitators which are useful in the anaerobic digester
include all known fluid agitators such as a turbine, propeller,
impeller, paddle, wheel, helical bar, stirrer, rotating reaction
vessel, flexible tube or rod, magnetic agitator, tumbler, paddle
wheel, and other mechanical agitators known to those of ordinary
skill in the art of fluid mixing. The preferred mechanical agitator
is a paddle.
By operating the anaerobic digester at higher pressures, higher
quality, i.e., purer, methane is produced. Generally, the higher
the digester pressure, the higher the purity or BTU rating of
methane produced by the reaction vessel. The anaerobic digester
generally does not require pressurization by external means as gas
formation in the digester tends to pressurize the reaction vessel
sufficiently. However, the reaction vessel can be pressurized with
a pressurizer. The pressurizer can be a compressed gas cylinder,
pump, or other such equipment, that forces an inert gas, a produced
gas, feedstock slurry, or reaction effluent into the reaction
vessel to increase the pressure of the reaction vessel to above
about 10 psi. Accordingly, the feedstock slurry feeder, gas
recirculator, fluid recirculator, sparger bar or combinations
thereof can serve as the pressurizer. In a preferred embodiment,
the anaerobic digester system will comprise one or more pressure
relief valves, vents or exhaust valves to reduce pressure within
the reaction vessel. The anaerobic digester will also preferably
comprise a pressure controller capable of controlling pressure
within reaction vessel and/or a pressure monitor capable of
monitoring pressure within the reaction vessel. The anaerobic
digester system can also comprise one or more pressure gauges that
indicate the pressure within the system.
The feedstock slurry feeder can be a force-feed or gravity-feed
system; however, a force-feed system is prepared. Preferred feeders
include pumps of all types or gas pressurized feed tubes or
chambers. Pumps are generally more preferred and a progressive
cavity pump is most preferred.
The productivity of the anaerobic digester system, in terms of gas,
especially methane, production is related to the pressure within
the reaction vessel. The present inventors have found that the
anaerobic digester can be operated at pressures exceeding 10 psi up
to about 265 psi. The increased pressure effects an increase in the
rate of gas, preferably methane, production and feedstock digestion
thereby reducing digestion periods and increasing the overall
productivity of the anaerobic digester system in terms of ft..sup.3
of methane produced per pound of feedstock. Generally, the higher
the pressure of the reaction vessel headspace, the higher the BTU
rating of the methane gas produced.
Temperature affects the productivity of the anaerobic digester.
Generally, elevating the temperature will increase the
productivity, e.g. faster or more efficient gas production, of the
digester up to a temperature that is harmful to the microbial flora
in the digester, at which temperature productivity will decrease.
Different microbes have different optimal temperatures. The
temperature of the reaction solution can be controlled with a
temperature controller that heats and/or cools the reaction
solution. The temperature controller can be a heater, heat
exchanger, jacket surrounding the reaction vessel, coil within the
reaction vessel or other such equipment used for controlling the
temperature of fluids within reactors. The temperature of the
reaction vessel will preferably be monitored with a temperature
monitor, such as a thermocouple or other equipment known to those
of ordinary skill in the art. Alternatively, the temperature of the
reaction solution is controlled by adding a temperature controller
to the fluid recirculator, the sparger bar, or the feedstock slurry
feeder. A heating or cooling jacket surrounding the reaction vessel
is alternatively used to control the temperature of the reaction
vessel contents.
Fluid levels in the reaction vessel are monitored with a fluid
level detector and controlled with a fluid level controller that
either increases or decreases the flow of feedstock slurry into or
reaction effluent out of the reaction vessel.
FIGS. 1a and 1b include a process flow schematic of a first
preferred embodiment of the anaerobic digester system according to
the invention. In this embodiment, cotton burrs obtained from a
cotton gin are converted to methane and a nutrient rich effluent.
Cotton burrs are separated from raw cotton in a cotton gin. The
processed cotton is baled and the cotton seed is collected. The
cotton burrs are collected and sized in a grinder to an acceptable
particle size to form a feedstock. The dirt and sand in the
feedstock are separated from the cotton burrs in a cleaner. The
cotton buffs are placed in a slurry mixer and heated fresh water is
added to form a feedstock slurry which is fed directly into the
digester. An anaerobic microbe is added to the digester to form a
reaction solution that is heated. The digestion period is allowed
to last for 1 to 60 days, preferably less than 45 days, and more
preferably less than 30 days, while forming a biogas containing
predominantly methane and carbon dioxide and possibly other gaseous
compounds. The biogas is passed through a scrubber to remove
unwanted components to form a raw gas mixture that is then passed
through a low-pressure compressor. The raw gas mixture from the
low-pressure compressor is collected in a low-pressure storage tank
or passed through a gas treater to remove carbon dioxide from the
raw gas and form high purity (>90% wt., more preferably >95%
wt, and even more preferably >98% wt.) methane. The high purity
methane is then compressed with a high-pressure compressor and
dried with a dehydrator to form "sweet-dry" methane. The sludge
from the anaerobic digester is sent to a collection tank or a dryer
to form dried sludge that can be used as landfill, a food
supplement, artificial peat moss, charcoal briquettes, fuel or
other similar purpose. The supernatant or effluent from the
anaerobic reactor contains ammonia and is sent to an ammonia
stripper that removes the ammonia from the supernatant. The treated
or ammonia-stripped supernatant is then fed back into the anaerobic
digester and used to digest additional feedstock. The ammonia
collected from the supernatant can be used to make a plant
fertilizer. Alternatively, a diluted form of the ammonia rich
supernatant is used as a fertilizer without removing the ammonia
therefrom.
Solid briquettes can be formed from the sludge by a process
including the steps of: a) removing the sludge from the digester;
b) optionally filtering the sludge in conventional solids
filtration equipment to remove the excess fluid from the sludge to
form a water-reduced sludge; c) forming the briquettes by pressure
molding the water-reduced sludge; and d) optionally drying the
briquettes in conventional drying equipment. Alternatively, the
sludge can be dried after either step a) or step b) above. The
briquettes and/or sludge need not be, but are preferably,
completely dried before use as a fuel.
FIG. 2 is a process flow diagram of a second preferred embodiment
of the anaerobic digester system according to the invention
comprising a feedstock source (16), a feedstock grinder (18), a
feedstock slurry tank (20) and mixer (19), a fresh water source
(17), a hot water gas-fired heater (6a), a hot water solar heater
(6b), a Jw/Dd-1 hot water heat exchanger (6c), an engine exhaust
hot water heat exchanger (6e), a discharge-gas hot water heat
exchanger (6d), a feedstock slurry feeder (23), an inlet port (2),
an anaerobic digester (1), a reaction solution agitator (comprising
a mechanical agitator (5a) and a sparger bar (5b)), effluent ports
(3a-3c), a supernatant storage tank (8), a sludge storage tank (9),
a scum storage tank (7), a discharge gas scrubber (10), a discharge
gas compressor (11), a discharge gas treater (12), a gas separator
(13), a discharge gas aerial cooler (14) and a discharge gas
storage vessel (15), a temperature controller coil (4), an optional
first gas recirculator (21), and an optional second gas
recirculator (22). The anaerobic digester (1) has a temperature
controller, e.g. heating coil, (4) through which water from the
heat exchangers or heaters is circulated to control the temperature
of the reaction solution. A sparger bar (5b) in the anaerobic
digester in combination with the recirculator (21) recirculates gas
from the headspace through the reaction solution to provide mild
agitation of the reaction solution. The mixer (5a) provides more
strenuous agitation if it is needed to mobilize the solids of the
reaction solution.
Under standard operating conditions, a feedstock is loaded into the
grinder (18) where it is ground to the desired particle size. The
ground feedstock and an aqueous solution are then placed in tank
(20) and mixed with the mixer (19) to form a feedstock slurry. An
inoculate of an anaerobic microbe and an aqueous solution is loaded
into the anaerobic digester (1). The feedstock slurry is loaded
into the anaerobic digester (1) with the feeder (23) through the
inlet port (2) until the desired amount of feedstock slurry is
added. The anaerobic digester is operated at an elevated pressure,
such as that generated by the digester itself, and at a temperature
sufficient to promote the digestion of the feedstock and the
formation of the product gas. After a sufficient period of time has
passed, the reaction solution is removed from the anaerobic
digester as a whole, in portions, or from different parts of the
digester. For example, if the reaction solution is permitted to
partition in the digester, the reaction solution can be drawn from
the scum, supernatant and/or sludge layers and placed in the
respective tanks (7,8,9). The product gas is removed from the
headspace of the digester (1) and passed through a gas scrubber
(10). Once the product gas is removed from the digester, it is also
termed the discharge gas. The discharge gas is then compressed by
the gas compressor (11) and passed through the gas treater
(12)--gas separation system (13) that removes CO.sub.2, H.sub.2 S,
and NH.sub.3 from the discharge gas. The gases removed from the
discharge gas can be passed through another gas separation system,
not shown, that isolates one or more of the gas components. The gas
separation system generally comprises a series of compressors,
condensers, evaporators, pumps, tanks and optionally heating and
/or cooling coils. The isolated component gas, preferably methane
for burning or CO.sub.2 for food grade CO.sub.2 production, is then
stored in the storage vessel (15) or released to a pipeline (not
shown).
The supernatant, sludge and scum solutions and slurries,
collectively termed the effluent, which are stored in their
respective tanks (7,8,9), are then used as nitrogen rich
fertilizer, insecticidal mixture, landfill, feed additive,
anaerobic digester inoculant, or other useful purpose. In addition,
the effluent can be dried using conventional equipment to form
valuable solid materials that can also be used as fertilizers,
charcoal, carbon black, feed additives or other useful materials.
The anaerobic digester tends to form ammonia, which can be removed
from the product gas by the scrubber (10), gas treater (12) or gas
separation system (13). The ammonia can be removed from the
effluent by evaporation, condensation, precipitation or reaction
with an acid source using methods well known in the art. The
effluent can also be filtered, centrifuged or placed in settling
tanks to separate the solids from the aqueous solution portion.
The product gas, if methane, can be used to operate gas-powered
machinery such as the hot water heat exchanger (6d), the water
heater (6a), and other equipment detailed above. Accordingly, the
anaerobic digester system of the invention can be used in a ranch
or farm setting to form a self-sustaining system. For example, the
anaerobic digester system can be operated in conjunction with a hen
house to convert waste from the hen house to methane gas, which is
used to operate machinery or equipment associated with the hen
house. A cooperative system as described will in effect permit
significant waste reduction thereby reducing the harmful effects
that excessive hen house waste has on the environment. According to
another example, the anaerobic digester system of the invention is
cooperatively in conjunction with a swine ranch to convert swine
waste to methane and a fertilizer solution, wherein the methane is
used to operate machinery used in the swine ranch. Other example
include the use of the anaerobic digester system in a cattle
feedlot or a dairy cattle operation to convert waste material to
methane and an insecticidal solution, wherein the methane is used
to operate machinery used in the feedlot or dairy ranch.
The anaerobic digester can be operated such that the reaction
solution is well mixed or stratified into the scum, supernatant and
sludge zones or layers. When stratified, the scum layer includes
materials that float in the reaction solution or are not well
digested by the anaerobic microbe in the digester. The sludge layer
includes materials that are denser than water and may or may not be
digested by the anaerobic microbe. The sludge can also include
feedstock solids that have not yet been digested. The supernatant
layer is between the scum and sludge layers and generally comprises
the bulk of the reaction solution. The supernatant layer includes
water soluble components of the feedstock slurry and water soluble
components, such as organic acids and ammonia, produced by the
anaerobic microbe or other microbe or enzyme catalyst present in
the digester.
The anaerobic digester system of the invention can be used to
digest feedstock comprising cotton burrs; cow manure; cow manure
mixed with feed, straw, hay, alfalfa, grass, soil, sand, tumble
weeds and/or wood shavings; and chicken manure and urine containing
wood shavings or cotton burrs. The useful materials prepared with
these feedstock materials include ammonia, methane, charcoal
(briquettes), carbon black, carbon dioxide, a nitrogen rich
fertilizer, a feed additive, an insecticidal solution, and/or an
insect repellant.
The supernatant solution taken directly from the reactor was tested
as an insecticidal solution. The supernatant was applied directly
to active fire ant mounds in a lawn. Within 24-48 hours, the ant
mounds were inactive. In some cases, no ant activity has been seen
in the treated mounds for a period of up to three to five months.
The surrounding treated lawn is lush and thriving.
Methane was prepared in the anaerobic digester system exemplified
herein. The product gas was collected from the headspace of the
digester and passed through a scrubber containing a mist pad, or a
glycol solution. The product gas was then compressed to about 300
psi. The gas was then passed through an amine gas treater to remove
CO.sub.2. The gas was then pressurized above 300 psi and passed
through a dehydrator to remove water to form sweet dry methane.
Finally, the methane was stored in a pressurized vessel for later
use.
Each of the sludge or supernatant layers prepared with the
anaerobic digester served as a nitrogen enriched fertilizer. For
example, an effluent solution that had been 80-90% digested was
applied to grass at the required rate. The effluent could be
diluted with water prior to application. Water containing effluent
was applied to a nearby patch of grass. Within about one to four
weeks, the treated grass was visibly greener and lusher than nearby
untreated grass.
Carbon dioxide is a common product of anaerobic digestion. The
carbon dioxide could be separated from the methane by use of a gas
separator. The isolated carbon dioxide can be used to make dry ice,
to pressurize the anaerobic digester or to provide an inert
atmosphere in the anaerobic digester. Alternatively, the carbon
dioxide can be reacted with caustic in the presence of heat or a
catalyst to form a bicarbonate salt. The carbon dioxide can also be
fed back into the digester to be converted to methane and increase
the overall yield of bioconversion to methane.
Ammonia is also a common product of anaerobic digestion. The
ammonia can be separated from the carbon dioxide and methane by
using a gas separator. The ammonia can be isolated as liquid,
compressed gas, or aqueous solution containing ammonia. For
example, when the product gas is treated with water in the scrubber
in a countercurrent manner, the water absorbs the ammonia from the
product gas thereby generating an aqueous solution containing
ammonia. Alternatively, the ammonia can be separated from the other
gases by treating the product gas with an acidic agent that reacts
with ammonia to form an ammonium salt or by passing the product gas
through a bed containing an sequestrant of ammonia to sequester the
ammonia. The ammonia can be used to make fertilizer or other
nitrogen-based products.
The sludge and scum layers contain relatively high concentrations
of solids. When the solids are dried, they can be used in
fertilizer, ground fill, pressed board material, solid fuel,
pressed fireplace logs, charcoal briquettes, medium for water
filters, or as a "peat moss" equivalent. Alternatively, the sludge
and scum layers removed from the reactor can be added back to the
reactor. The solids may also be blown into a burner system, such as
a boiler or furnace, for use as fuel. The sludge can be dried by
air, sun and/or heat.
EXAMPLE 1
Anaerobic Digestion of Chicken Manure
This process was conducted in a batch-type manner. Fresh well water
(318 gal.) was loaded into a digester equipped with an internal
mechanical agitator and a heat controller. The water was heated
until it reached a temperature of about 100.degree. F. Then,
anaerobic bacterium inoculant (10 gal.) and fresh wet chicken
manure (14.5 lbs.) were added to the water with mixing. Finally,
clean unground cotton burrs (200 lbs.) were added to the digester
with mixing and the digester was sealed. The digester was then
purged repeatedly with nitrogen gas to create a substantially
anaerobic environment. With this loading, the percent solids of the
reaction solution was approximately 4.48%. The digester was run for
a period of 45 days with periodic sampling of the headspace. The
temperature ranged from about 95.degree. F. to about 120.degree. F.
and averaged about 105.degree. F. to about 110.degree. F. The
pressure within the digester ranged from about 1 psi to about 45
psi. The total amount of gas produced was about 931.5 ft..sup.3.
The average rate of gas production was about 0.88 ft..sup.3 /hr,
about 0.004 ft.sup.3 /hr/lb. of burrs or about 4.66 ft..sup.3 /lb.
of burrs. The individual gas components of the product gas ranged
from about 49% wt. to about 65% wt. for methane and from about 51%
wt. to about 35% wt. for carbon dioxide. The content of ammonia in
the product gas was not measured.
The following analytical measures were determined for the cotton
burrs and chicken manure used in this procedure.
CHICKEN DRIED SOLIDS MEASURE BURRS MANURE TOTAL IN EFFLUENT Carbon
(lbs.) 16.0789 1.16 17.2389 Nitrogen (lbs.) 0.429 0.1239 0.5529
Carbon/Nitrogen 37.48 9.39 31.18 Energy 7593 7330 (BTU/lbs.) Ash (%
wt.) 6.00 10.91 7.22 Volatile 76.30 16.52 69.33 Solids (% wt.)
CHICKEN DRIED SOLIDS MEASURE BURRS MANURE TOTAL IN EFFLUENT
Moisture (% wt.) 17.49 71.5 23.45 Nitrogen (% wt.) 0.21 1.03
For other 200 lbs. batches using approximately the same amounts of
ingredients, the total C/N ratio ranged from about 30 to about 32.
In other batches run according to this procedure, the percent
solids ranged from 4.5% to 5.0% wt.
EXAMPLE 2
Anaerobic Digestion of Chicken Manure
This procedure was substantially the same as that of Example 1. The
digester produced product gas containing about 63% methane.
EXAMPLE 3
Anaerobic Digestion of Cow Manure
This process was conducted in a batch manner. Fresh well water (90
gal.) was loaded into a digester equipped with an internal
mechanical agitator and a heat controller. The water was heated
until it reached a temperature of about 100.degree. F. Then, a
Clostridium spp. inoculant (1 gal.) and cow manure slurry (60 gal.)
were added to the water with mixing to a vessel having a total
capacity of 150 gal., and the digester was sealed. The digester was
then purged repeatedly with nitrogen gas to create a substantially
anaerobic environment. With this loading, the percent solids of the
reaction solution was approximately 25-50%. The pH of the digester
slurry was optionally adjusted to about 6.5-6.8 with lime. The
digester was run for a period of 80 days with periodic sampling of
the headspace. The temperature ranged from about 70.degree. F. to
about 100.degree. F. and averaged about 80.degree. F. to about
85.degree. F. The pressure within the digester ranged from about 5
psi to about 18 psi. The pH of the reaction solution was kept
between about 6.5-6.8 by the addition of lime. The methane produced
was vented each day and the amount collected ranged from about
20-40 ft..sup.3 and averaged about 30-40 ft..sup.3. The total
amount of methane produced was about 2000 ft..sup.3. About 10-15
lbs. of feedstock were added on a semi-weekly basis. The total
amount of feedstock added was about 300 lbs. The amount of sludge,
scum and supernatant removed from the reactor was about equal to
the amount of feedstock added. The average rate of gas production
was about 6-7 ft..sup.3 of methane/lb. of feedstock. FIG. 3 depicts
a chart of the measurements obtained for reaction solution pH,
amount of methane gas produced (ft..sup.3), reaction solution
temperature (.degree. F.), and the headspace pressure of the
reactor (psi). The BTU rating of the methane produced by the
digester operated under these conditions ranged between about 600
to about 850.
The above is a detailed description of particular embodiments of
the invention. It is recognized that departures from the disclosed
embodiments may be made within the scope of the invention and that
obvious modifications will occur to a person skilled in the art.
Those of skill in the art should, in light of the present
disclosure, appreciate that many changes can be made in the
specific embodiments which are disclosed herein and still obtain a
like or similar result without departing from the spirit and scope
of the invention. All of the embodiments disclosed and claimed
herein can be made and executed without undue experimentation in
light of the present disclosure.
* * * * *